In a nuclear carbon/oxygen ratio (C/O) well logging tool, a sealed tube neutron generator (STNG) is employed to provide 14 Mev fast neutrons(n) by the nuclear interaction d+t=.alpha.+n. The existing conventional STNG, for example, disclosed by CN patent No. 2052573u, consists of gas-tight insulator electrodes, a gas-tight envelope, Penning ion source, gas supply, accelerating electrode and target. Bombarding the formation with 14 Mev neutrons, prompt nelastic gamma rays of carbon (4.43 Mev) and of oxygen (6.13 Mev) are induced. By measuring the gamma ray spectrum, the atomic carbon/oxygen ratio and consequently oil saturation in the formation should be obtained under ideal conditions.
However, the C/O values logged by logging apparatus that employs a conventional STNG have large errors. The error is induced mainly by the cased-hole materials (includes cased hole fluid) that contain a large amount of the elements C, O, S and Ca. The .gamma. rays emitted from them constitute a very high background for the gamma spectrum logged. Moreover, the .gamma. rays from thermal neutron capture(capture-.gamma.) also constitute the background for the inelastic .gamma.-spectrum. These backgrounds adversely interfere with the C/O ratio logged. Although the method and apparatus that employs conventional STNG have been improved recently, the problem remains unsolved.
CN patent application No.93109244 disclosed a novel Carbon/Oxygen Well Logging Apparatus which adopts an Associated-Particle and Time of Flight Approach for collecting data. The apparatus will be called APTF-C/O-WLA hereinafter. Referring to FIG. 1A, the APTF-C/O-WLA comprises a STNG with a .alpha.-signal acquiring device 9 (.alpha.-STNG), fast timing electronics 25, BaF.sub.2 .gamma. detector 17 and other conventional members that are used in conventional well logging apparatus. The .alpha.-signal acquiring device 9 comprises a plurality of small .alpha.-detectors that surround the incident ion beam; each small .alpha.-detector consists of a scintillator, a light guide, and a photo multiplier tube. It has been tested and verified in well logging that the APTF-C/O-WLA is able to resolve the above-mentioned problem of using conventional well logging apparatus, and to improve the precision of the C/O logged significantly. The working principle is introduced as follows:
Referring to FIG. 1A and FIG. 1B, by the T(d,n)He nuclear reaction caused by the bombardment of the deuterium ions on the target, a 3.6 Mev .alpha.-particle and an associated 14 Mev neutron are produced simultaneously and emitted in opposite directions. The direction of the .alpha.-particle specifies the subsequent trajectory of the individual neutron. When the neutron emitted forward is scattered inelastically with the materials (both cased-hole material 18 and formation 16) surrounding .gamma.-scintillator 17, a gamma ray is emitted. The cross sections of the cased-hole material 18 and detectable formation 16 are two adjacent annuluses. Thus mounting an annular .alpha.-detector 9 before the target, and by using the Associated Particle and Time of Flight Approach, the background of the .gamma. spectrum can be eliminated. This is explained as follows: Assume that Tt represents the total time of flight of a neutron and the .gamma. induced by the neutron. Because the speed of .gamma. is much faster than the speed of the neutron, then the Tt may represent the time of flight of neutron Tn approximately, and subsequently represents the distance from the target to position P where the .gamma. is induced by the neutron. The equal-value surface of Tn may be convened to a sphere approximately. In FIG. 1A Tl and Tr represent two equal-value surfaces of Tn for predetermining the formation region to be logged. In FIG. 1B, the output signals of .alpha. detector 9 are shaped by constant-fraction discriminator (CFD) and routed through Delay to the start-input of a Time to Amplitude Convener (TAC); the .gamma.-rays generated by inelastic neutron interactions are converted to an energy spectra with .gamma.-detectors 17 and 19, the fast timing signals are shaped by CFD and routed through Delay to the stop-input of the TAC to provide a range-proportional, Time-of-Flight Spectrum (TOF) at TAC output (Tt). The TOF spectrum includes the contributions from the time-of-flight of the .alpha. particle and the electronics; but if the contributions are approximately constants, the TOF spectra represent approximately the relative time-of-flight spectra of the neutrons. Using SCA to select the Tl and Tr at the TOF spectrum, the signals from the portion between Tl and Tr correspond to the .gamma. rays that occur within the formation regions of interest. From viewing their longitudinal section, the regions will be that surrounded by curved lines Tl, Tr and ray lines T.sub.D ' and T.sub.C ', and that surrounded by curved lines Tl, Tr and ray lines T.sub.A ' and T.sub.B '. The time signals mentioned above are routed to the gate input of multiple channel analyzer (MCA) to turn on the time gate, the energy signals in the .gamma. energy spectra are routed to the E-input of MCA, so that only the energy signals of .gamma. rays that occur within the predetermined formation region are recorded by MCA. The .gamma. rays that occur within the cased-hole materials are not recorded at all. The reason is that a part of them has no associated-.alpha. particle (occurring within the region between rays T.sub.B ' and T.sub.D '), and another part of them occurs out of the region predetermined by Tl and Tr (within the region to the left of Tl). On the other hand, the procedure relative to capture-.gamma. is a slow procedure being of the order of 10 .mu.s; the procedure relative to inelastic prompt .gamma. is a fast one being of the order of ns; the main point of the associated particle and time of flight approach used here is fast coincidence method; its time resolution is equal to the difference of Tr-Tl, which usually is about 8 to 10 ns. So the capture-.gamma. induced by a thermal neutron can not be counted in MCA inevitably. It just can be counted occasionally.
Obviously, the key element of the APFT-C/O-WLA is the .alpha.-STNG, but the above CN patent application has not disclosed its specific structure. There will be described in this specification that a .alpha.-STNG suitable for using in APTF-C/O-WLA for well logging has particular structure rather different from that of a conventional STNG.
Particularly, the above mentioned CN Patent application has not disclosed the integral structure of the .alpha.-STNG that can be used with the above well logging apparatus which adopts Associated-Particle and Time of Flight Approach, the structure of the parts relative to ions beam, and the specific standards for designing and manufacturing the .alpha.-signal acquiring device and the relatively small .alpha.-detectors associated therewith. However, in order to manufacture a qualified .alpha.-signal acquiring device, the designers have to obey some particular and specific standards, which are given as follows:
(1) The associated-.alpha. detector should have the following features:
1. Possessing satisfactory geometric characteristics: that is, having wide enough sterad for accepting associated .alpha. particles; capable of distinguishing the cased-hole materials from the formation region to be logged; the optimized and predetermined formation region to be logged being located nearby and surrounding the .gamma.-scintillator; its whole body being suitable to be used in the limited space of a cased hole well.
2. Possessing satisfactory signal characteristics: that is, output signals having high enough magnitude for distinguishing noise and other interference; output signals having short enough rise time for decreasing the uncertainty of timing; capable of providing satisfactory equal time surface of neutron flight time.
3. Possessing satisfactory manufacture process.
(2) The associated .alpha. detector usually comprises a scintillator, a light guide (if needed) and a photo multiplier tube assembly.
1. The scintillator of the .alpha.-detector in the .alpha.-STNG is a very thin layer of material, so it needs a transparent-rigid substrate for supporting it. The process for manufacturing the scintillator is: Depositing a thin layer of inorganic scintillation phosphor (e.g., ZnS, ZnO) on the surface of the selected substrate; covering it with very thin layers of an organic membrane and of Al-film on it sequentially; then heating them to remove the organic membrane: and finally vapor plating a layer of Al-film to protect the scintillation phosphor from the bombarding of scattered deuterium ions. The layers of scintillation phosphor and Al-film are called the .alpha.-scintillator. The manufacturing process requires the substrate to have an open surface, for example, a flat, or the surface of a frustum of a circular cone with large opening angle. If the surface of the substrate is the inner surface of a glass tube, it will be very difficult to make a qualified .alpha.-scintillator. The .alpha.-scintillator must have a sufficient area and satisfactory sterad for accepting .alpha. particles.
2. The area of the photo cathode of the photo multiplier tube should be as close to the area of the .alpha.-scintillator as possible. The count rate of .alpha.-signals accepted by each photo multiplier tube may be reach 2.times.10.sup.5 CPS to 3.times.10.sup.5 CPS; such high count rate requires the photo multiplier tube to have wide enough area of its cathode, for example, its diameter may be longer than 20 mm. To enable the photo multiplier tube to provide a high enough magnitude of signals, the photo multiplier tube must have many dynodes; this leads to the photo multiplier tube having a rather long length, which may be more than 50 mm. Based on the description above and the existing manufacture process for making high temperature photo multiplier tubes, a suitable and available photo multiplier tube assembly may have a volume larger than 24 mm diameter by 60 mm .
3. Referring to FIG. 2, the dimensions and position of the optimized formation region to be logged depends on the distances from the target to the center point of .gamma.-scintillator 16 (usually S is longer than 300 mm ), the outer diameter of cased-hole well W (usually W is about 200 mm ), and the detectable depth of formation region F. The geometric characteristics of an .alpha.-detector depends on its outer diameter D, inner diameter d, the height H of the scintillator, the diameter of target t and the average distance L between the .alpha.-scintillator and the center point of the target; If the .alpha.-scintillator has three geometric degrees of freedom (D, d and H), it is easy to achieve satisfactory geometric characteristics by adjusting the D, d, H and L. For example, its shape may be similar to the outer surface of a frustum of a circular cone. If the .alpha.-scintillator has two or less geometric degrees of freedom, it is difficult to achieve satisfactory geometric characteristics; for example, its shape is similar to the inner surface of a hollow cylinder of glass. In this case, when both d and H take smaller values, the .alpha.-scintillator may have wider sterad, but the formation region to be logged will be far away from the .gamma.-detector. When S takes a value of 300 mm and W takes 200 mm , in order to get suitable sterad and formation region to be logged, the outer diameter of .alpha.-scintillator D has to be larger than 50 mm . The inner diameter of steel housing G is usually smaller than 90 mm . Therefore, there will be no sufficient space surrounding the hollow cylinder of glass to accommodate suitable photo multiplier tubes.
(3). The Associated-Particle and Time of Flight Approach demands the diameter of target 12 to be less than 12 mm ; and due to the needs of mounting target chamber and multiplier photo multiplier tubes, the ion beam transporting length in the .alpha. STNG is much longer than that in a conventional STNG; so the .alpha.-STNG needs an assembly with a special structure for leading out, focusing and transmitting the ion beam.
Moreover, the structure of the .alpha.-signal acquiring device 9 in the .alpha.-STNG 14 disclosed by the above mentioned CN patent application is rather complex; the manufacture processes thereof is rather difficult; the internal structural materials thereof are rather massive, and more gas is easy to be released during its storage and usage periods. Thus, the working life of the .alpha.-STNG is short. Therefore, further improvements are needed.
Another CN patent, No. CN 205273u disclosed an STNG incorporating an a scintillator for well logging (referring to FIG. 3). The STNG comprises an .alpha.-scintillator 9 welded between a target assembly 12 and a conventional STNG without target. The .alpha.-scintillator 9 is made of a hollow cylindrical tube of glass over whose inner surface a ZnS scintillator is smeared. However, this patent did not disclose how to assemble photo multiplier tubes with .alpha.-scintillator to get a really usable STNG, as there is no sufficient space surrounding the .alpha.-scintillator for accommodating suitable photo multiplier tubes, and there is no proper manufacturing process revealed for this kind of .alpha.-scintillator. Therefore, the STNG cannot be used in the APTF-C/O-WLP to satisfy the special requirement of well logging.
Moreover, the existing conventional-STNG and .alpha.-STNG have a problem that their internal high vacuum suffers from the leakage of air and the release of gases from its internal parts, inevitable during both storage and usage. Consequently its effective working life is shortened.